Conventionally, there have been used inkjet printing apparatuses that perform printing on a printing medium by scanning, relative to the printing medium, an ejection mechanism in which a plurality of outlets for ejecting minute ink droplets are arranged. In recent years, inkjet printing apparatuses have also been used for printing where the content to be printed varies for each page (so-called variable data printing) such as in credit card bills or the like, in which case, for example, printing is continuously performed on both the front and back surfaces of a plurality of pages set for a long-length printing medium.
Along with such printing apparatuses, various techniques for inspecting a printing defect where the content printed on the front surface of a page does not correspond to the content printed on the back surface of that page (i.e., the content to be printed on the back surface of one page is printed on the back surface of a different page) have been proposed. For example, Japanese Patent Application Laid-Open Gazette No. 2001-58446 discloses a technique for use in an inkjet printer that performs color printing on both the front and back surfaces of a long-length printing medium, in which an identifier such as a number, a symbol, or a bar code is printed on the front and back surfaces of each page, and a printing defect where the contents printed on the front and back do not correspond to each other is inspected by capturing and comparing images of the identifiers on both the front and back surfaces on the downstream side of the conveyance direction of the printing medium.
Meanwhile, like Japanese Patent Application Laid-Open Gazette No. 2001-58446, Japanese Patent Application Laid-Open Gazette No. 2004-314610 discloses a technique for use in an electrophotographic printing apparatus, in which a printing defect where the contents printed on the front and back do not correspond to each other is detected by printing a bar code or the like on both the front and back surfaces of a printing medium and capturing and comparing images of the bar code or the like on the downstream side. The printing apparatus disclosed in Japanese Patent Application Laid-Open Gazette No. 2004-314610 also enables detection of a printing defect where the front and back surfaces of a certain page are both missing, by comparing the bar codes on the front surfaces of adjacent pages and also comparing the bar codes on the back surfaces of these pages.
Incidentally, with the printing apparatuses as disclosed in Japanese Patent Application Laid-Open Gazettes Nos. 2001-58446 and 2004-314610, the bar codes on the front and back of each page are printed with a black (K) ink that can be read by an ordinary bar code reader. Therefore, a printing apparatus that performs color printing cannot detect printing defects due to inks of colors other than black even if such printing defects have occurred.